Il 07/07/2014 16:35, Dr. David Alan Gilbert ha scritto:
* Paolo Bonzini (pbonz...@redhat.com) wrote:
Il 04/07/2014 19:41, Dr. David Alan Gilbert (git) ha scritto:
From: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
Postcopy needs to have two migration streams loading concurrently;
one from memory (with the device state) and the other from the fd
with the memory transactions.
Can you explain this?
I would have though the order is
precopy RAM and everything
prepare postcopy RAM ("sent && dirty" bitmap)
finish precopy non-RAM
finish devices
postcopy RAM
Why do you need to have all the packaging stuff and a separate memory-based
migration stream for devices? I'm sure I'm missing something. :)
The thing you're missing is the details of 'finish devices'.
The device emulation may access guest memory as part of loading it's
state, so you can't successfully complete 'finish devices' without
having the 'postcopy RAM' available to provide pages.
I see. Can you document the flow (preferrably as a reply to this email
_and_ in docs/ when you send v2 of the code :))?
From my cursory read of the code it is something like this on the source:
finish precopy non-RAM
start RAM postcopy
for each device
pack up data
send it to destination
and on the destination:
while source sends packet
pick up packet atomically
pass the packet to device loader
(while the loader works, userfaultfd does background magic)
But something is missing still, either some kind of ack is needed
between device data sends or userfaultfd needs to be able to process
device data packets.
Paolo
Thus you need to be able to start up 'postcopy RAM' before 'finish devices'
has completed, and you can't do that if 'finish devices' is still stuffing
data down the fd.
Now, if hypothetically you had:
1) A migration format that let you separate out device state so that you
could load all the state of the device off the fd without calling the device
IO code.
2) All devices were good and didn't touch guest memory while loading their
state.
then you could avoid this complexity. However, if you look at how Stefan's
BER code tried to do 1 (which I don't do in my way of doing it), it was by
using the same trick of stuffing the device data into a dummy memory file
to find out the size of the data. And I'm not convinced (2) will happen
this century.
Paolo
Dave
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
